Their models showed unprecedented increases in drought, heatwaves and flooding, with some areas set to face intensification of more than one hazard. The team made continent-wide comparisons possible for the first time by applying a consistent method.

Under the highest impact scenario, no city is immune from stronger and more frequent heatwaves by the end of the century, the researchers found. Although southern Europe may see more heatwaves per year, central European cities will be affected by the most dramatic temperature increases – of up to 14°C. For the first time, the team also brought to light possible worsening of drought in northern cities.

The increased risk of flooding to the British Isles was one of the report’s most striking findings. Even under the most optimistic scenario, 85% of UK cities are projected to face more flooding. Cities including Cork, Carlisle and Wrexham could see more than a 50% increase in peak river flow. In the worst-case scenario, only 9 cities in the study are immune to a greater flood risk.

Previously, projections of river flooding changes for Europe have been inconsistent. By using a longer analysis interval of 50 years, and one-in-ten year, rather than one-in-100 year, floods, Selma Guerreiro and colleagues teased out a strong north-south divide.

The team used results from climate model runs forced with a high greenhouse gas emissions trajectory known as Representative Concentration Pathway (RCP) 8.5, which assumes a forcing of 8.5 W/m2 by the year 2100. Under this scenario, the emissions curve begins to flatten only after mid-century.

"We used RCP 8.5 because we wanted to assess the impact of climate change in European cities if severe emission reductions do not happen," said Guerreiro. "The work started in 2013, when there was no international agreement in place. However, it is still relevant since the national commitments from the Paris agreement imply a global temperature increase of around 3 degrees, well within the temperature range covered by RCP8.5."

The response of the Earth system to climate forcing is still uncertain. To explore the full range of possible outcomes, the team used the whole ensemble of climate projections from the fifth Coupled Model Intercomparison Project (CMIP5), presenting the data in low (10%), medium (50%) and high (90%) impact categories. These represent the percentiles for each hazard for each city, indicating the range of outcomes rather than probability.

The team was particularly concerned about the findings for dramatic increases in drought in southern Europe. They describe a picture of "a fundamentally different climate" for the region, a scenario that present infrastructure may not be prepared for.

Now the group is combining its work on hazards with research into vulnerability. This will build up a complete picture of risk under unmitigated climate change, helping decision makers prioritise investments for adaptation.